Support version 3.0 of the Compose file format

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

tests/fixtures/v3-full/docker-compose.yml

@@ -0,0 +1,37 @@
+version: "3"
+services:
+  web:
+    image: busybox
+
+    deploy:
+      mode: replicated
+      replicas: 6
+      labels: [FOO=BAR]
+      update_config:
+        parallelism: 3
+        delay: 10s
+        failure_action: continue
+        monitor: 60s
+        max_failure_ratio: 0.3
+      resources:
+        limits:
+          cpus: '0.001'
+          memory: 50M
+        reservations:
+          cpus: '0.0001'
+          memory: 20M
+      restart_policy:
+        condition: on_failure
+        delay: 5s
+        max_attempts: 3
+        window: 120s
+      placement:
+        constraints: [node=foo]
+
+    healthcheck:
+      command: cat /etc/passwd
+      interval: 10s
+      timeout: 1s
+      retries: 5
+
+    stop_grace_period: 20s